Ultra-violet B light is a spectrum of light that is invisible to us but is visible to insects and some other organisms. In humans it causes suntan and sunburn and is implicated in the formation of eye cataracts. It is the light emitted by tanning bulbs.

UVB light also affects marijuana potency. The potency of high quality marijuana increases in direct ratio to the amount of UVB light it receives. This is very significant. In California, where the medical dispensaries operate in an unrestricted market; many dispensaries reject fall harvested outdoor material as inferior. They have found it lacks the potency of indoor crops and is a harsh smoke. However, when they were presented with marijuana grown outdoors but forced to ripen August 10, they accepted it as if it were indoor because of its high potency and lack of harshness. I think the harshness results from cool nights.

Indoors, under fluorescent and HPS lamps, gardens receive little UV-B light. Metal halides emit a bit more. However, there are ways of supplying your garden with UV-B light. Tanning lamps work, that is, lamps that tan people, because of the UV-B light they emit. Using tanning lamps will increase the THC content of the crop. Reptiles and lizards require the spectrum to stay healthy. So the spectrum usually comprises about 10 percent of their output. If you want to try tanning lamps they are available on the Internet. Use between 5-10 percent of your total wattage to these lamps. For a 1000-watt garden use 100 watts of special lighting.

"Third, the MH-lamps emit more UVB light than HPS lamps, although still in very small amounts. The amount of UVB light plants receive is directly related to the quality of the buds. The more UVB, the higher the quality." Ed Rosenthal